In one prior art about the pneumatic screw nail gun, a main air valve is installed between the pneumatic motor and the air diving path of the cylinder. The main air valve serves to control the actuation of the pneumatic motor and the cylinder in the gun body at the same time so that the nail locking rod can driving rod and descend to provide a twisting force to the screw nail so that the screw nail can be beaten into enter into the work piece. A cruciform portion at a front end of the nail locking rod is engaged to a cruciform groove in the screw nail. Thereby, the screw nail can be beaten into the work piece. Thus, the nail locking rod is locked.
However, in above prior art pneumatic screw nail gun, the main air valve serves to control the actuation of the pneumatic motor and the driving path of the cylinder in the gun body. Although the object of locking the screw nail is achieved, no device for accumulating air pressure in air driving path of the pneumatic motor is installed. As a result, when the pneumatic motor is driven by air pressure, the output twisting force is unstable. Especially, when a react force is generated because the screw nail is beaten into a work piece, the dynamic power of the pneumatic motor will reduce. Therefore when the air supplied to the pneumatic motor will be unstable. Thereby, the input air pressure cannot work with the downward pressing of the cylinder. Then the operation of the screw nail gun cannot be well controlled.